CMPT 212 Object-Oriented Applications Design in C++ Fall 1997

A Simple MS-Windows Program (continued)

• There are 5 steps to producing a window:
  1. Define the window class.

  2. Register it with the OS: RegisterClass().

  3. Create an instance of the window class: CreateWindow().

  4. Display the window: ShowWindow().

  5. Update the contents of the window: UpdateWindow().

  (Note that the words "class" and "instance" are used in a general sense, and not in the specific object-oriented meaning.)

The Message Loop

• Any messages sent to a window are first sent to the application which owns the window.
  • The application has the opportunity to act on these messages.

  • The application usually passes the messages on to the appropriate window using the DispatchMessage API function.
• MS-Windows maintains queues for messages, since messages may be generated faster than they can be processed.

• Here is a diagram showing the interaction between the application and MS-Windows, the routing of messages, and the role of the message loop:

• The function GetMessage blocks until a message is available.
  • It returns a non-zero value if the message is not the "Quit" message (WM_QUIT).

The Callback Function

• The purpose of the callback function is to handle messages dispatched from the message loop.

• A message structure (MSG) has a field which contains the handle (HWND) of the window in which the event occurred.
  • MS-Windows puts the message in the queue for that window.

  • When it gets time, MS-Windows will call the callback function for that window so that the message can be processed.

• Normally a callback function contains just a switch statement where each case handles a different message.

Outputting Text to a Window

• The output statement

     cout << "Hello";
           

  will not work. It does not specify:
  • which window to display in,
  • where in the window to display,
  • which font to use,
  • which colour to use.

• The position of text output is specified by using X-Y coordinates relative to the top-left corner of the window, not to the screen.

• Suppose we want to display "Here" at the location where the user presses the left mouse button.
  • Here is the code for this:

       LRESULT CALLBACK WindowFunc(HWND   hwnd,
                                   UINT   message,
                                   WPARAM wParam,
                                   LPARAM lParam) {
          HDC  hdc;
          int  X;
          int  Y;
          char str[80];
    
          switch (message) {
    
             ...
    
             case WM_LBUTTONDOWN :
                hdc = GetDC(hwnd);
                strcpy(str, "Here");
                X = LOWORD(lParam);
                Y = HIWORD(lParam);
                TextOut(hdc, X, Y, str, strlen(str));
                ReleaseDC(hdc);
                break;
    
             ...
    
             }
    
          return 0;
       }
     
                   

• The HDC variable is a handle to a device context.
  • A device context describes the output device:
    • printer vs. screen,
    • number of colours available,
    • font and colour to use,
    • clipping or no clipping,
    • and many others.

  • GetDC returns the default device context. It you want to change the font, colour, etc., you must change it after calling GetDC but before using the device context.

  • You must release the device context before returning from the callback function.

• TextOut does not erase text.
  • To erase text, you would have to write spaces at the location of the text.

  • But remember that if you are using a variable pitch font, then you may require more spaces than the number of characters you are trying to erase.

Return to lecture notes index

Lectures